Although microwave ovens have been utilised for rapid heating domestically for several years, it is only recently that their use for batchwise organic synthesis has been reported.sup.21-24. It has been demonstrated that microwave heating can provide a means for dramatically increasing the rate of reactions. In the above synthetic organic chemical publications.sup.21-24 the microwave reactions were carried out in sealed containers usually made of polytetrafluoroethylene (PTFE). Under those conditions, the solvent was rapidly superheated and high pressures developed in the containers, thus enabling the reaction mixtures to reach temperatures greater than those attained at reflux. The rate enhancements observed were attributed.sup.23,24 to these effects of temperature. However severe disadvantages are associated with the published technique.
The reactions have been carried out in sealed vessels often surrounded by vermiculite and housed in an outer container. Such a system prevents observation of the mixture during the reaction and precludes adequate mixing for reactions involving more than one phase. Conventional temperature sensors being metallic can cause arcing and so cannot be used in a microwave oven. There is also no way of measuring the pressures generated in the sealed systems. Reactions thus cannot be carefully monitored and can easily be overheated, creating the potential for overreaction, decomposition and even explosions.
Hot reaction mixtures have also been found to transfer heat to the PTFE containers, softening them. The pressure in the reaction vessel has then caused the walls of the container to deform and rupture explosively on occasions. According to the Canadian group.sup.23 this tended to occur when reaction times were increased in an attempt to reach completion. To try to overcome this problem, Gedye et al..sup.23 employed PTFE containers with pressure-release caps and recommended that these be filled to only 10-15% capacity and operated at reduced power settings and for limited times (up to 5 minutes). These modifications appear to present fresh difficulties. A potential safety hazard would be created by venting flammable solvents (and reactants) directly into the oven and only low volumes of materials could be reacted at any one time under limited conditions.
Another recognised disadvantage is that the rate of reaction also decreases as the volumes of reagents and solvent increase.sup.1, thus substantially militating against scale-up and restricting the method to small batches.
A flow through microwave catalysis system has been disclosed by Wolf et al..sup.25. This known system uses an industrial 2.5 GHz Micro-Aire Oven with a variable power level of 80-1000 watts and a glass reaction cell. In this known system a metal catalyst within the reaction cell is heated by microwaves applied in pulses with appropriate off-time periods to keep the overall temperature of the reaction cell at a desirable range.
U.S. Pat. No. 3,535,482 to JH Kluck discloses microwave apparatus for rapidly heating fluids, namely foods such as fruit juice, soup, puree, etc. for blanching, concentrating, pasteurisation and sterilization. This known apparatus provides a flow through system for continuous processing in which heat is generated directly within the fluid and in which the flow conditions and pressure of the fluid are controllable. The apparatus includes a length of microwave transparent tubing, through which the fluid passes, mounted transversley through a waveguide. As described, the length of the tubing exposed to the microwave energy is made as short as possible such that the amount of fluid being heated at any instant is at a minimum. This requirement imposes restrictions on the diameter of the tube and its location within the waveguide, depending on the nature of the fluid, desired outlet temperature and the frequency of the microwave energy used for the heating. Thus, unlike the apparatus of the present invention, the apparatus described in U.S. Pat. No. 4,535,482 does not permit of precise control, in any one application, of a range of parameters suitable for carrying out chemical reactions on a continuous basis. Furthermore, the requirement that the tube passing through the waveguide be of minimum length and the concommittent short residence time (described as being in the order of 0.1 to 0.01 seconds) of the fluid within the heating zone, would cause the apparatus to be unsuitable for conducting chemical reactions. Also, in this known apparatus the heated fluid is depressurised and vented to atmosphere prior to being cooled, which is an arrangement in which volatile products would be lost were the apparatus to be used for carrying out a chemical reaction.